The present invention relates to the field of electronic circuits, and more particularly, to an electronic temperature sensor.
Temperature sensors are used in a wide variety of applications. Many different types of temperature sensors are commercially available, and the type of temperature sensor that will be used in any particular application will depend on several factors. For example, cost, space constraints, durability, and accuracy of the temperature sensor are all considerations that typically need to be taken into account.
One particular application in which a relatively high degree of accuracy may be required of a temperature sensor is in the field of antennas. More particularly, so-called xe2x80x9csmartxe2x80x9d antenna systems are commonly being used in both ground based applications (e.g., cellular antennas) and airborne applications (e.g., airplane or satellite antennas). Smart antenna systems, such as adaptive or phased array antennas, combine the outputs of multiple antenna elements with signal processing capabilities to transmit and/or receive communications signals. As a result, such antenna systems can vary the transmission or reception pattern of the communications signals in response to the signal environment to improve performance characteristics.
Of course, one of the factors which affects the signal environment is the temperature at which the antenna elements operate. Accordingly, to provide accurate phase shifting in a phased array antenna system, it is generally desirable to know the temperature of the antenna elements
Typical prior art temperature sensors may include thermistors, resistance-temperature detectors (RTDs), and active temperature-dependent current sources, for example. One such active temperature-dependent current source is the AD590 by Analog Devices, Inc., of Norwood, Mass., which is further described in the data sheet entitled xe2x80x9cTwo-Terminal IC Temperature Transducerxe2x80x9d from Analog Devices, Inc., published 1997. Yet, in typical prior art temperature sensor configurations, such devices may require a connection to additional circuitry such as multiplexors, analog conditioning circuitry, and analog-to-digital (A/D) converters, for example.
This additional circuitry not only increases the cost of the temperature sensor, but may also require a relatively large amount of space. Furthermore, to provide a high degree of accuracy, such sensors typically require careful calibration over the operating temperature environment. This may be particularly difficult to perform in spaceborne antennas, for example, where operating temperatures may vary significantly depending upon whether the antenna elements are shaded or in direct sunlight.
Because of issues such as cost, space savings, and the difficulty of calibration, many phased array antenna systems include only a single centralized temperature controller coupled to temperature sensing devices such as those listed above. For example, U.S. Pat. No. 5,680,141 to Didomenico et al. entitled xe2x80x9cTemperature Calibration System for a Ferroelectric Phase Shifting Array Antennaxe2x80x9d discloses a phased array antenna that includes a single temperature sensor circuit connected to a plurality of temperature sensors, each of which senses the temperature of a phase shifter separate from the phased array antenna elements. Each phase shifter is connected to a plurality of antenna elements. The temperature sensor circuit connects to a data processor system for inputting temperature information used to calculate calibration error factors.
One drawback of such phased array antennas is that all of the temperature compensation processing is performed by a central processor. Thus, if temperatures of a large number of phase shifters are to be monitored, the controller""s task of managing temperature compensation may become significantly complicated and require a significant amount of processing resources. Communicating analog temperature data from a large number of sensors back to a central processor can also require a significant amount of wiring and analog processing.
In view of the foregoing background, it is therefore an object of the present invention to provide a relatively accurate temperature sensor that may be easily calibrated.
This and other objects, features, and advantages in accordance with the present invention are provided by a temperature sensor including a capacitor, a circuit element coupled in series with the capacitor and having a resistance that varies with temperature, and a controller. The controller is for charging/discharging the capacitor through the circuit element, measuring a charging/discharging time required to charge/discharge the capacitor to a predetermined threshold, and determining a temperature based upon the charging/discharging time.
More specifically, the circuit element may be a thermistor, for example. The temperature sensor may further include at least one calibration resistor coupled between the controller and the capacitor. As such, the controller may sequentially charge/discharge the capacitor through the circuit element and the at least one calibration resistor, measure respective charging/discharging times required to charge/discharge the capacitor to the predetermined threshold through the circuit element and the at least one calibration resistor, and determine the temperature based upon the charging/discharging times. In particular, the at least one calibration resistor may include a high and a low calibration resistor.
Additionally, the controller may include a counter for measuring the charging/discharging time, a driver coupled to the circuit element for charging/discharging the capacitor, and a control logic circuit for controlling the driver. Further, the controller may also include a Schmitt hysteresis device coupled to the capacitor for determining when the capacitor has been charged to the predetermined threshold. The controller may also advantageously be implemented in an ASIC.
A method aspect of the invention is for sensing temperature using a capacitor and a circuit element having a resistance that varies with temperature. The method may include charging/discharging the capacitor through the circuit element, measuring a charging/discharging time required to charge/discharge the capacitor to a predetermined threshold, and determining the temperature based upon the charging/discharging time.
More specifically, the circuit element may be a thermistor, for example. The method may also include coupling at least one calibration resistor to the capacitor. As such, charging/discharging the capacitor may include sequentially charging/discharging the capacitor through the circuit element and the at least one calibration resistor, and measuring the charging/discharging time may include measuring respective charging/discharging times required to charge/discharge the capacitor to the predetermined threshold through the circuit element and the at least one calibration resistor. Moreover, determining the temperature may include determining the temperature based upon the charging/discharging times.
Additionally, the at least one calibration resistor may include a high calibration resistor and a low calibration resistor. Measuring the charging/discharging times may further include measuring the charging/discharging time using a counter, and charging/discharging the capacitor may include coupling a driver to the circuit element and charging/discharging the capacitor using the driver.